Effect of Ionic Strength on Solvation Geometries in Aqueous Nitrate Ion Solutions

Temperature-dependent infrared spectra of aqueous nitrate solutions with a range of concentrations and ionic strengths are used to determine the effect of ionic strength on the relative stabilities of different solvation geometries in aqueous nitrate ion. The asymmetric stretching absorption band from 1250 to 1450 cm^-1 changes line shape with temperature, allowing two distinct peaks to be fit for each spectrum. Because each peak is assigned based on electronic structure calculations as a proxy to a different solvation motif, van't Hoff plots provide insight into the thermodynamics of exchange between different solvation geometries. A strong linear trend is seen between increased ionic strength and the magnitude of both the enthalpic and entropic contributions to the solvation geometry stabilities. Electronic structure computations of previously proposed structures in different ionic strengths are performed in the presence of external fields, roughly simulating the impact of ions in solution.